This invention pertains to a drain apparatus, particularly a drainmast for aircraft, and particularly to a modular drainmast including a drain tube removably enclosed in a fairing that is removably attachable to an aircraft.
A drainmast is used to eject waste water from an aircraft waste system during flight without impingement of the ejected fluid further aft on the body of the aircraft, and to drain the waste system when on the ground.
As used herein the term "waste water" means undesired water-based liquids that accumulate during operation of an aircraft on the ground and in flight. Waste water is generated by many sources, one common source being the aircraft galley in which water, soft drinks, coffee, wine, orange juice and other potable liquids are collected. Waste water can also accumulate from engine and other air inlets that trap water from condensation or ingest rain.
A drainmast generally comprises a drain tube enclosed within an aerodynamically shaped, drag minimizing fairing that when installed protrudes downwardly from the underside of an aircraft such that its outlet extends into the surrounding airflow stream during flight. It may be swept aftwardly of the aircraft with the direction of airflow. The drain tube is coupled to the aircraft waste water storage tank and is heated to avoid freezing. In certain applications waste water is accumulated until it reaches a certain volume. Thereupon a valve inside the aircraft opens and the waste water drains from the tank through the drainmast. In other applications, the waste water is continuously drained.
There are two prevalent configurations for drainmasts. One is herein referred to as a bottom discharge mast. The other configuration is herein referred to as a rear discharge mast.
In a bottom discharge drainmast, the waste water is ejected generally perpendicularly to surrounding airflow. The mast is generally straight and swept. The axis of the drain tube at the exit forms an acute angle with the axis at the entrance. The angle is usually close to the angle of sweep of the mast.
In a rear discharge drainmast, the mast is swept and wing shaped and tapers into an exit tube at the tip that is roughly parallel to surrounding airflow. Water is ejected parallel to the surrounding airflow from the rear of the mast. Since the axis of the drain tube at the exit is nearly perpendicular to the axis at the entrance, the drain tube is either bent in a smooth curve or is fabricated from two or more straight sections that together form the appropriate angle.
The rear discharge mast is believed to be preferable to the bottom discharge mast. A portion of the water ejected from a bottom discharge mast has a tendency to adhere to the tip of the mast and dribble back driven by surrounding airflow. Much of it can freeze to the mast before it is blown away. In contrast, a properly designed rear discharge mast generates a suction action as surrounding airflow passes around the exit tube. Ejected water is sucked out of the tube into surrounding airflow. Very little if any water adheres to the mast.
Drainmasts have been used on aircraft for many years. A common design developed for early commercial jet aircraft and still in use today is a rear discharge configuration and includes a fairing consisting of two shaped aluminum halves that are fastened together with screws. Each fairing half has a channel that receives the drain tube. The drain tube is formed from stainless steel or copper. Silicone elastomer tape is wrapped around the tube in several locations. The tape compresses when the halves are screwed together and firmly locates the tube in the mast. Because clearance between the tube and the mast is very small, considerable heat loss to and through the fairing occurs. The drain tube is either bent or fabricated from multiple sections. The drain tube cross section is oddly shaped ranging from oblong to triangular. Some designs have a cross section that changes between the inlet and the outlet. This design is suspected of being more susceptible to clogging than drain tubes of constant cross section. The drain tube is locally heated by fixed resistance ceramic type resistors. Heating along the length of the drain tube is provided only by heat conduction along the tube. A heated portion of the drain tube extends out of the drainmast at the tube exit to prevent freezing of residual water. Sustained temperature of the drain tube can reach 700.degree. F. The exterior of the drainmast can reach temperatures on the ground which present a hazard to service personnel. Some designs are open at the top where the drainmast mates with the aircraft. This can present a fire hazard if hydraulic fluid or lubricating oils leak into the drainmast and contact the heating elements.
Another known drainmast design is of bottom discharge configuration and has a fairing that is formed from two pieces of compression molded fiber reinforced plastic. The two halves are fastened together with adhesive and screws. The drain tube is copper, has a constant round cross section, and is attached to a shaped metal tip that covers the end of the mast. The heating element is wire or foil encapsulated in silicone rubber. The heating element is bonded to the outside of the tube and inside the metal foot that covers the end of the drainmast. The heated metal foot is required to prevent discharged waste water residue from freezing to the drainmast. As discussed previously, this is an inherent problem with bottom discharge drainmasts. The drain tube is sandwiched between the fairing halves which are mated to the metal foot. The entire assembly is filled with expanding foam. The resulting unit is integral and cannot be disassembled without damaging the components. The top of the unit is sealed where it mates with the aircraft. A controller inside the aircraft cycles power to the heating element to maintain temperature within preset limits. Power is transferred to the drainmast via a connector that is attached to the mounting base of the mast.
A more recent known drainmast design also utilizes a fiber reinforced plastic fairing. Mirror image fairing halves are formed from resin impregnated fabric that is laid into a female mold, vacuum bagged, and cured under heat and pressure, and trimmed after molding. The drain tube is formed from fiber reinforced plastic and has an integral heating element encapsulated in fiber reinforced plastic and bonded to the outside of the tube. The fairing halves are bonded together with the drain tube sandwiched in between. Lightweight filler material such as polyurethane expanding foam occupies the void in between the tube and fairing. The resulting drainmast is integral and cannot be disassembled without damage or destruction of the drainmast assembly. A heated foot is not required because the rear discharge feature is utilized.
The heating element for this drainmast design is a pattern of wire that has a high positive coefficient of electrical resistivity. As the element heats the resistance increases which causes supply current to decrease. A steady power consumption and drain tube temperature is reached the values of which depend on the operating environment of the drainmast. This feature is referred to as a self regulating heating element. An electrical controller inside the aircraft is not required which greatly simplifies the system. A thermoswitch may be located inside the drainmast. The thermoswitch is operably connected to cause opening of the power supply circuit when the thermoswitch temperature is above a preset value to prevent current from flowing to the heating element when outside air temperature is above freezing. This feature is desirable to prevent overheating the fiber reinforced plastic fairing. It also serves to decrease overall power consumption of the drainmast. All electrical components are encapsulated inside the drainmast and cannot be removed.
Yet another recent known drainmast design has a fairing consisting of fiber reinforced plastic that is resin transfer molded in one piece. As in other known embodiments the drain tube and thermoswitch are integrally molded in place inside the fairing during the resin transfer molding process and cannot be removed without damage or destruction.